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Full text of "Handbook Of Chemical Engineering - I"

246                               CHEMICAL ENGINEERING
to a position where the bed of particles is barely or slightly submerged. An up and down motion given to the hand screen is not so effective as the alternation of this movement with a sidewise shake. The up and down or jigging motion tends to bring the coarse particles to the screen while the fine go to the top and it is also very effective in keeping the bed loose so as to get interstitial effect while giving the screen sidewise strokes. On the down movement of the jigging stroke the bed must be submerged and on the up movement raised slightly out of the water to get the draining away which assists in carrying the undersize grains through the aperture. The side stroke to give interstitial effect must be done with the bed submerged. With power screens owing to the extra mechanical complexity added in obtaining a jigging stroke inventors have so far contented themselves with a flat shaking screen either only slightly submerged in the water or submerged during some part of the stroke. With the first arrangement waves are set up in the tank on which the screen is mounted and water gets trapped above the screen and the bed of grains becomes matted down and waterproof. This difficulty increases as the size of material fed diminishes. When the screen is only submerged during part of the stroke the matting of the bed of grains is greater. The other difficulty which occurs with wet screening is the disposal of the products made by the screen. If the undersize be allowed to flow from the bottom of the tank on which the screen is mounted the opening provided for this purpose must be sufficiently large to prevent clogging and this will require excessive amounts of water. If mechanical means are provided to remove the undersize it adds to the complexity of the design. If a flat horizontal screen be employed, it is evident that much of the water will spill over the end on the tank on which it is mounted from the wave motion set up by the action of the screen unless some special form is used. Thus J. M. Callow's patent (788,246, April 25, 1905), shows a shaking screen with a small upturned hinged portion at the discharge end, and it seems a screen with an upturned curved portion at the discharge end would give the same effect.
Wet screening with flat screens either placed horizontally or inclined and using water in the form of sprays does not give satisfaction. The water drains away very quickly, leaving a balled up sticky mass which is difficult to progress over the screen, unless an excessive amount of water is used the sprays reaching every part of the screen.
J. M. Callow has experimented with and devised a number of wet screens but the net results of his labor is the endless-belt machine with spray near the discharge end. The screen belt is mounted on the drums which support it with sufficient slack so that it droops and assumes a slight curve between the entry point of the material fed at the rear drum and the forward drum where the sprays are placed. The washing back effect at this point affords the chief opportunity for the grains to pass over the apertures. At the entry point only the water and small undersize goes through the screen the balance of the material fed matting down upon it. While the Callow screen has no movement effective in securing good and rapid movement over the screen cloth over the screen nevertheless it has attained commercial importance. The size range of the machine is from 16 to 200 mesh. Below 16 mesh the wires of the cloth are HO stiff that they do not pass readily around the drums. Cloth finer than 80 mesh is usually too costly to use with this device.
Where capacity is the desideratum rather than precision of screen work the governing points are: The width of the screen and the rate at which it advances the material over it. In screens designed rather for capacity than closeness of work the load per unit width of the screen and the rate of advance of material are more important factors than the length of the screen or the area of it. For good screen work the stroke meaning here the actual distance the material advances under a pulse of the actuating mechanism the forward movements of the material fed being usually less than those of the actuating motion and tending to decrease as the load on the screen